The discovery of what has been termed a "subterranean Galapagos" was announced by the Deep Carbon Observatory Tuesday, which said many of the lifeforms have lifespans of millions of years.
DCO executive director Robert Hazen said the findings are the "crowning achievement" of the 1,000-strong collective of scientists, who "have opened our eyes to remarkable vistas -- emerging views of life that we never knew existed."
The biomass of the organisms' ecosystem is estimated at 15 to 23 billion metric tonnes (16.6 to 25.4 billion tons), which is hundreds of times greater than that of all human life, and comprises a volume of 2 to 2.3 billion cubic kilometers (480 to 550 million cubic miles) -- almost twice that of all the planet's oceans.
Making use of advances in technology, scientists drilled 2.5 kilometers (1.5 miles) into the seabed and sampled "microbes from continental mines and boreholes more than 5 km deep," a report from the observatory says.
The findings look set to change our ideas about life on Earth and have implications for the likelihood of similar discoveries on other planets.
There are millions of distinct types of bacteria as well as archaea -- microbes with no membrane-bound nucleus -- and eukarya -- microbes or organisms with cells that contain a nucleus and have membrane walls -- living beneath the Earth's surface, the report says, possibly exceeding the diversity of surface life. Around 70% of the planet's bacteria and archaea are now thought to live underground.
"Deep microbes are often very different from their surface cousins, with life cycles on near-geologic timescales, dining in some cases on nothing more than energy from rocks," the report says.
DCO co-chair Mitch Sogin said the discovery was like finding a new Amazon.
"Exploring the deep subsurface is akin to exploring the Amazon rainforest. There is life everywhere, and everywhere there's an awe-inspiring abundance of unexpected and unusual organisms."
The abundance of subsurface life has opened a metaphorical can of worms.
Among the questions that the discovery poses is how the abundant life began, and how it survives and reproduces in extreme conditions.
"Did life start deep in Earth (either within the crust, near hydrothermal vents, or in subduction zones) then migrate up, toward the sun?" the report asks.
"Or did life start in a warm little surface pond and migrate down? How do subsurface microbial zombies reproduce, or live without dividing for millions to tens of millions of years?"
It also asks questions of how this life spreads, and what its main source of energy is.
"How do the absence of nutrients, and extreme temperatures and pressure, impact microbial distribution and diversity in the subsurface?" it asks.
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